The present invention relates to electrochemical systems. More particularly, the present invention relates to non-liquid proton conductors such as solid polymer proton conducting membranes, for use in electrochemical systems, under ambient conditions.
An electrochemical system includes two electrodes, referred to as cathode, where reduction occurs during use, and anode, where oxidation occurs. When electrons flow through an electrical circuit from one electrode to the other, i.e., according to the above definitions from the anode to the cathode, charge is equalized by movement of ions from one electrode to the other via the electrolyte.
To this end, in most electrochemical systems the electrodes are separated therebetween by an aqueous solution, referred to as an electrolyte, through which ions can freely move.
However, as it is not always convenient to have a liquid present in an electrochemical system, systems were developed, wherein a non-liquid electrolyte is employed for proton conduction. For proton conduction, non-liquid electrolytes are non-liquid proton conductors typically in the form of an organic polymer or an inorganic material. For various uses of non-liquid electrolytes in electrochemical systems the reader is referred to U.S. Pat. No. 3,265,536 to Miller et al., U.S. Pat. No. 4,664,761 to Zupancic et al., U.S. Pat. No. 5,272,017 to Swathirajan et al., U.S. Pat. No. 4,594,297 to Polak et al., U.S. Pat. No. 4,380,575 to Nakamura et al., U.S. Pat. No. 4,024,036 to Nakamura et al., U.S. Pat. No. 4,089,816 to Sano et al., U.S. Pat. No. 4,306,774 to Nicholson, and U.S. Pat. No. 4, 179,491 to Howe et al.,
Since electrochemical processes are advantageously run at elevated temperatures, and as heat can be produced during the electrochemical process, these non-liquid electrolytes have to be heat resistant.
Nevertheless, there are many electrochemical applications which are run at room temperature, i.e., ambient temperature, and due to size or current use, do not produce much heat.
For use at elevated temperatures up to one hundred .degree.C., a familiar organic material is a Du-Pont product under the name of Nafion, which contains fluorinated methanesulfonic acid groups giving it its thermal stability. An example of an inorganic material frequently used in this range of temperatures is hydrogenuranylphosphate.
However, for ambient conditions these materials are not very convenient, as they are very expensive and do not excel in ionic conductivity at room temperatures. Therefore, their activity is boosted by working at higher temperatures and pressures, where currents per unity of area are maximized and as a result, less area of the expensive membrane is necessary. For use under ambient conditions, commercially available organic polymer ion exchange sheets are typically employed as non-liquid electrolytes. These however are expensive, unstable and have the additional disadvantage of a bad electrical contact with the electrodes, which at ambient temperatures is more of a hindrance than at elevated temperatures.
In order to evade these problems, use of heterogeneous systems, where an insoluble ion exchange material is mixed with a polymer, or alternatively, use of homogeneous systems, where acids like sulfuric, phosphoric or heteropolyacids are dissolved in a polymer, were initiated. Nevertheless, the former still have the disadvantage of bad electrical contact with the electrodes, while in the latter, the acidic material tends to leach out.
There is thus a widely recognized need for, and it would be highly advantageous to have, a non-liquid proton conductor for use in electrochemical systems under ambient conditions, which systems are characterized by (i) an electrical contact between the non-liquid proton conductor and the electrodes, which is as good as that obtained using liquid electrolytes; and (ii) a proton conductor which by nature does not leach out.